Protective coupling circuit



oclt. 25, 1949.

Filed June 27, v 1944 A. G. KANDOIAN PROTECTIVE COUPLING CIRCUIT 2v Sheets-Sheet 1 I N VEN TOR. fz/fM/G G. /ff//vpa/fm/ Oct. 25, 1949. A. G. KANDOIAN 2,485,606

PROTECTIVE COUPLING CIRCUIT Filed June 27, 1944 2 Sheets-Sheet 2 -S Lo LO o) d A. m l b Q Q I -wr lLI t 3X L@ 5g m i i o l l 'i o o i I l Patented Oct. 25, 1949 UNITED vS'I`.\1`ES PATENT OFFICE PROTECTIVE COUPLING CIRCUIT Armig G. Kandoian, New York, :N Y., assignor to Federal Telephone and Radio Corporation, New York, N. Y., a corporation of Delaware Application June Z7, 1944,*Serial No. 542,278

' (Cl. -Z50-13) 7 Claims.

:My finvention relates to improvements in proftective circuits, and more particularly to arrangements for coupling both a transmitter and a re- 'ceiverito a common antenna so as to `prevent high transmitter voltages from entering the receiver.

In my co-pending application, Serial Number l"469,174 led December 16, 1942, I have disclosed la protective circuit wherein a gap is bridged across *the midpoint of a half-wave impedance trans- .for-mer in the receiver-transmission line. This :system is satisfactory for fairly wide frequency lband operation but does not provide a high resonant voltage stepup for the gaps. Although yhigh resonant voltage stepup for the gaps is desirable `for longgap life and satisfactory receiver protection, in the usual resonant sections the selectivity is very sharp and hence not satisfactory for wide frequency band operation.

It is accordingly an object of the present inven- 'tion `to provide means whereby the blockingA of high transmitter voltages from a receiver connected to a common antenna is possible over a relatively wide frequency range. Another object ofthe present invention is to provide high resoriant impedance points for the gaps, to insure maximum voltage stepup for protective discharge gaps over the desired frequency range.

Another object of the present invention is to provide 'a protective circuit which is particularly adapted 'for'use with transmitter-receiver systems employing coaxial transmission lines.

A further object of the present invention is to .provide a protective circuit for transmitter-re- 'ceiver `systems which will cause little or no mismatch Yin the line even at the limits of the operating frequency band.

Other objects and advantages of the invention will fbe apparent from the following description of aipreferred form of the invention and from the drawing, in which:

Fig. l1-is a schematic diagram of a transmitterreceiver system incorporating a preferred form of fthe present invention; and

Fig.v2.is,a curve showing the degree of mismatch .introduced into the receiver-transmission line of Fig. 1 'over a predetermined band of frequencies.

The main purpose of the preesnt invention is to permit the 'use of a common antenna by a transmittel and receiver over a relatively wide band of frequencies, while at the same time automatically preventing the high transmitter voltages from being impressed upon the receiver. Fig. 1 illustrates altransmitter I and areceiver 2 adapted to be con- .n'ected to a common antenna 3. Transmitter I is A2 receiver' 2 ris connected to a :coaxial transmission line 5. Lines d and 5 are joined at point A, from which point a common coaxial transmission line It leads to antenna 3.

:Connected .in xthe transmitter-transmission line 4 :isfan impedance-matching yl-ine section `I w'lfiicll 'also-:serves to block received lenergy fromltransnditter il in a manner hereafter to be described. r`lll-iis aline section 1 comprises a sectionof coaxial tine open 2at the far end where a spark gap '-T iis placed. When the transmitter is on, the lgap II breaks down so that sectiony -'I offers little -disturbance to the flow of energy yto 4the antenna. When however the antenna -is used for reception, .the received signal lbeing small does not break down the gap 'I'. The impedance across point lIIB becomes substantially Zero, thus preventing any of the received energy from going into the transmiltter.

Connected .in the Areceiver-transmission line f5 4is a=blocking network 8, this ynetwork 8 fconsisting of two spaced-apart coaxial line vsections 9 and 10. Inner conductors II and I2 of line vsections 9 and II) fare respectively fconnected tothe 'inner conductor of receiver-transm'ission 1ine5 atpoints 'C and 5D, :these connections preferably being ad- 'justa'ble -so that the position Aof points 1C and lD along inner conductors II 4and I2 respectively may 'ibe nvaried. Likewise -the junctions -betw'een line 5, and the outer conductors of *lines A'TSI-and IlJ are @preferably Asimilarly adjustable, so that -rthese sections Sand Il) -may be moved as a whole ywith respect tol line 5.

nnerconductcrs llll `and I2 of -coaxial line sections 9 and I each have one end thereof connected itc the `closed lends I3 vand IlI of their respective line sections. The otherends'of the linner conductors II and I2are each separated from the closed ends I5 and 'I6 of their respective 'line sections by space discharge devices fsuch as gaps -II fand I8 respectively.

The present Ainvention is designed 'to :be effective over a predetermined band of frequencies. The length of each of the coaxial line sections 9 and I0 is accordingly made substantially Iequal to one-quarter wavelength at the mid-"frequency of this band, and the spacings A-B, -A-C and C-D are also each made equal substantially lto one-'quarter wavelength at this mid-frequency. Asaresult, `when the transmitter I is in operation energy transmitted on all frequencies between the limits of this predetermined kband is substantially 'precluded from reaching the lreceiver 2. How this is brought about wlill nowbe connected `toa coaxial transmission line iII, while 55 described.

Without the protective features of the present invention, energy from transmitter I would divide at point A, a portion being radiated from antenna 3 and the remaining portion being conducted by transmission line to receiver 2 to result in damage to the receiver or other undesirable eifects. According to the present invention, however, this portion of the transmitter energy is substantially blocked from the receiver 2 by the network 8.

It is known that by moving the point of connection of an energy source from the closed end toward the open end of a stub line section, the frequency response of the line section may be broadened. That is, by connecting the source at or near the closed end, the section will be resonant at substantially only a single frequency. However, by moving the energy input connection toward the open end, operation becomes less critical, and the section may be made to resonate over a relatively wide band of frequencies.

For the purpose of illustration, let it be assumed that the range over which the system of the present invention is designed to operate covers the band between 500 and 600 megacycles. It was stated above that the length of each of the line sections 9 and III is chosen to be substantially one-quarter wavelength at the mid-frequency of this band, or 550 megacycles. If the distance between points C and D and closed ends I3 and III respectively of line sections 9 and I0 be designated as 01, and if the distance between points C and D and closed ends I5 and I6 respectively of the line sections be designated as 02, then and a decrease in the value of 02 will effect a broadening of the frequency band over which line sections 9 and I9 will be resonant. If 02 is now chosen so that line sections 9 and I0 as a unit will be resonant over the band between 500 and 600 megacycles, the energy from transmitter I on any frequency within this band will cause a voltage transformation along line sections 9 and IU, as is well-known in the art. In other words, the voltage at gaps I'I and I8 will be much higher than at points C and D in receiver-transmission line 5.

The dimensions of gaps I'I and I8 are so chosen that the voltage built up in the manner above described exceeds the breakdown voltage of the gaps. When breakdown occurs, an effective short circuit across receiver-transmission line 5 results, and this eiective short circuit acts to block the transmitter voltages from receiver 2. It should be noted that due to the one-quarter wave spacing between points A and C, the eiective short circuit at point C results in line 5 presenting a relatively high, substantially infinite, impedance at point A. The blocking action of network 3 will thus have no appreciable effect on the transmission of energy between transmitter I and antenna 3.

The reactance presented by line section 9 at point C is generally primarily inductive. Since point D is a one-quarter wavelength from point C, the reactance presented by line section I0 is primarily capacitive. Therefore line sections 9 and I0 together act like a parallel coil-condenser circuit of high Q.

An alternative mode of operation in the example given is to adjust 62 so that line section 9 will resonate more or less sharply at 500 megacycles, and so that line section I Il will resonate more or less sharply at 600 megacycles. Energy from transmitter I on a 500 megacycle frequency will then break down gap I 1, while the voltage transformation of non-resonant line section I0 is insuicient to break down gap I8. At 600 megacycles, gap I8 only would break down. By selecting proper values of 02, and choosing proper dimensions for gaps I1 and I8, energy on an intermediate frequency within the band may be made to break down either or both gaps II and I8 according to its departure from the mid-frequency of the band.

It is common in connection with a transmitter and receiver connected to the same antenna to block the operation of the receiver during the operation of the transmitter. In Fig. 1 is shown a blocking circuit I9 that may be used for this purpose if desired.

In the above description I have illustrated how the transmitter voltages are blocked from receiver 2. However, the system of the present invention is also effective in permitting energy received by antenna 3 to be conducted to receiver 2 with a minimum of attenuation when the transmitter I is not operating.

Under receiving conditions, energy picked up by antenna 3 is conducted by transmission line 6 to point A, where it would normally divide to flow over both transmission lines 4 and 5. However, due to the action of line section 1, an effective partial short circuit is produced in transmission line 4 at point B, a quarter-wavelength from point A. The partially short-circuited line section between A and B accordingly presents a relatively high impedance to energy incoming over line 6, blocking in part the flow of energy to transmitter I. Accordingly most of the received energy is applied to receiver 2.

The energy received by antenna 3 passing through network 8 is of a value much lower than the value of the energy from transmitter I. Consequently, the breakdown voltage of gaps II and I8 is not reached, no blocking occurs, and the received energy is conducted without appreciable loss to receiver 2.

Fig. 2 illustrates the relatively low degree of mismatch that is introduced in transmission line 5 by the action of blocking network 8, the curve covering the band between 500 and 600 megacycles which has been used above as an illustration. It will be observed that the line is substantially flat between 530 and 580 megacycles, and that even at the limits of the frequency band the standing wave ratio does not exceed approximately 2 to 1.

For best results, the surge impedance of the line sections 9 and I 0 should be greater than the surge impedance of the transmission lines. For example, if a standard 50 ohm transmission line is employed, the line sections 9 and I0 may have an impedance in the order of ohms.

While the space discharge devices II and I8 have been illustrated, by way of example, as spark gaps, it will be clear to those skilled in this art that any other space discharge devices such as neon tubes may be employed in place thereof. Also, while the invention has been described in connection with coaxial lines, it will be clearly understood that other types of transmission lines such as open wire lines may be employed if desired.

Moreover, While the receiver and transmitter.

are shown coupled to a common antenna, it is clear that they may be coupled to some other form of utilization circuit such as a common carrier transmission line, without departure from the scope of my invention.

While I have described above the principles of my invention in connection with a specic system, it will be clearly understood that this description is made only by way of example and not as a limitation on the scope of my invention as set forth in the objects and the accompanying claims.

I claim:

1. In combination, a transmitter, a receiver, a common utilization circuit for said transmitter and receiver, a first transmission line leading from said transmitter to said utilization circuit, a second transmission line interconnecting said receiver and said first transmission line, and a protective network in said second transmission line, comprising a pair of spaced-apart resonant stub lines open-circuited at one end and short-circuited at the other coupled to said second line at a point intermediate their ends, and means forming a space discharge device between the conductors of each of said stub lines at the respective opencircuited ends thereof.

2. A combination according to claim 1, wherein one of said resonant stub lines is spaced from the junction of said second transmission line and said utilization circuit substantially a quarter Wavelength at the operating mid-frequency of the system.

3. In a system of the type in which a receiver and a transmitter operating over a relatively wide band of frequencies are both coupled to a common antenna, said transmitter and receiver being connected to said antenna by a first transmission line and a second transmission line respectively, the combination of a protective network in said second transmission line to substantially block high transmitter voltages from said receiver, comprising a pair of spaced-apart resonant line sections open-circuited at one end and| short-circuited at the other, the sharpness of the resonance curve of each section depending on the distance between the open-circuited end of the section and the point of connections between the section and said second transmission line, and means comprising a space discharge device between the open-circuited ends of the conductors of each of the sections.

4. A system according to claim 3, in which said two line sections are each resonant at the mid-frequency of the said operating band.

5. A system according to claim 3, in which said two line sections are resonant at the respective frequency limits of the said operating band.

6. A system according to claim 3, in which said two line sections are spaced apart along said second transmission line a distance equal to approximately one-quarter Wavelength at the midfrequency of the operating band.

'7. A system according to claim 3, in which one of said two line sections is spaced approximately one-quarter wavelength at the mid-frequency of the operating band along said second transmission line from the junction between said first and second transmission lines, and the other of said two line' sections is spaced approximately onequarter wavelength at the mid-frequency of the operating band along said second transmission line from said one line section.

ARMIG G. KAN'DOIAN.

REFERENCES CITED The following references are of record in the le of this patent:

UNITED STATES PATENTS Number Name Date 1,073,624 Pickerill Sept. 23, 1913 2,401,717 Wolff et al June 4, 1946 2,412,161 Patterson Dec. 3, 1946 2,412,315 Brown Dec. 10, 1946 2,425,379 Lindenblad Aug. 12, 1947 2,438,367 Keister Mar. 23, 1948 

